1. Technical Field
The present invention relates to a charging device used for electrophotographic and electrostatic recording processes, and an image forming apparatus using the charging device.
2. Related Art
In image forming apparatuses using electrophotographic systems, a uniform charge is formed on an image supporter (photoreceptor), an electrostatic latent image is formed thereon using a laser beam in which an image signal is modulated, and the electrostatic latent image is developed into a toner image with charged toner. Subsequently, the toner image is electrostatically transferred to a recording medium directly or through an intermediate transfer body to obtain a desired transferred image.
As described above, in an image forming apparatus using an electrophotographic system, charging treatment is carried out to form a uniform charge on the image supporter. An example of the charging member for such charging treatment is a contact-type charging member. Such contact-type charging members have advantages in that they usually apply an smaller electric current and produce significantly smaller amounts of ozone as compared with non-contact type charging members such as corotrons or scorotrons.
Contact-type charging members comprise an electro-conductive support having formed thereon a layer of an electro-conductive elastic body. The charging member is abutted against a photoreceptor in an electrophotographic apparatus, and a predetermined voltage is applied between the charging member and the photoreceptor to apply a charging potential to the photoreceptor. In contact-type charging members, precise resistance control in the semiconductive region is indispensable for achieving both uniform charging ability for uniformly charging a photoreceptor and leak resistance which prevents the concentration of electric current at a pinhole (a minute defect such as a small diameter hole) generated on a photoreceptor. In contact-type charging members, charging rolls having a roll shape are widely used.
It is known that in electrophotographic apparatuses using a charging roll, the surface potential of a photoreceptor can be more uniformly charged by superimposing a peak-to-peak alternating voltage that is at least two times greater than a breakdown voltage on a direct current voltage.
Recently, there has been a demand for longer operating life in electrophotographic apparatuses. The operating life of electrophotographic apparatuses is limited in particular by the wear of photoreceptors. For reducing the wear of photoreceptors, it is necessary to enhance the strength of the photoreceptor surface against wear, or reduce stresses which accelerate the wear.
Examples of methods proposed for the former treatment include imparting wear resistance to the photoreceptor surface, and forming a surface layer having excellent wear resistance.
For the latter treatment, when a contact-type charging member is used for charging a photoreceptor surface, particularly when charging is carried out by superimposing of an alternating current voltage, a method to reduce the applied alternating current voltage (electric current) is suggested.
However, if a sufficient alternating current voltage is not applied, satisfactory uniform charging effect by the superimposing of the alternating current voltage cannot be achieved, which results in image defects such as density irregularity due to non-uniform charging.